It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic or magnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone" the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers are deactivated upon checkout by a deactivation device which changes an electromagnetic or magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
One type of magnetic EAS system is referred to as a harmonic system because it is based on the principle that a magnetic material passing through an electromagnetic field having a selected frequency disturbs the field and produces harmonic perturbations of the selected frequency. The detection system is tuned to recognize certain harmonic frequencies and, if present, causes an alarm. The harmonic frequencies generated are a function of the degree of non-linearity of the hysteresis loop of the magnetic material.
Another type of EAS system employs magnetomechanical markers that include a magnetostrictive element. For example, U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a marker formed of a ribbon-shaped length of a magnetostrictive amorphous material contained in an elongated housing in proximity to a biasing magnetic element. The magnetostrictive element is fabricated such that it is resonant at a predetermined frequency when the biasing element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an ac magnetic field at the predetermined frequency, and the marker mechanically resonates at this frequency upon exposure to the field when the biasing element has been magnetized to a certain level.
According to one technique disclosed in the Anderson et al. patent, the marker has, in addition to the aforesaid resonant frequency, an "anti-resonant frequency" at which the stored mechanical energy resulting from magneto-mechanical coupling is near zero. An interrogation circuit which provides the magnetic field at the interrogation zone is swept through a frequency range that includes the marker's resonant and anti-resonant frequencies, and receiving circuitry is provided at the interrogation zone to detect the marker's characteristic signature by detecting a peak transmitted energy level which occurs at the resonant frequency, and a valley level at the anti-resonant frequency.
Anderson et al. also propose that the magnetostrictive element be subjected to annealing over a period of 7-120 mins. at a temperature in the range of about 300.degree.-450.degree. C. in the presence of a saturating transverse magnetic field of a few hundred oersted to enhance a magneto-mechanical coupling factor k which is related to the difference in frequency between the resonant and anti-resonant frequencies of the marker. According to Anderson et al., a larger coupling factor k increases the detectability of the marker's characteristic signature.
In still another surveillance system proposed by Anderson et al., a magnetostrictive marker is used with an interrogation frequency that is not swept, but rather remains at the marker's resonant frequency. The interrogation field at this frequency is provided in pulses or bursts. A marker present in the interrogation field is excited by each burst, and after each burst is over, the marker undergoes a damped mechanical oscillation. The resulting signal radiated by the marker is detected by detecting circuitry which is synchronized with the interrogation circuit and arranged to be active during the quiet periods after bursts. EAS systems of this pulsed-field type are sold by the assignee of this application under the brand name "Ultra*Max" and are in widespread use.
For markers used in pulsed-interrogation systems, the amplitude and duration of oscillations which the member continues to exhibit after the end of each excitation pulse are very important. The greater the amplitude and duration of the residual oscillations (known as "ring down"), the more unique is the signal during the quiet period in the interrogating zone and therefore the easier it is for the marker to be detected by the detecting circuitry.
Deactivation of magnetomechanical markers is typically performed by degaussing the biasing element, so that the magnetostrictive element ceases to be mechanically resonant or its resonant frequency is changed. However, when the biasing element is degaussed, although the marker is no longer detectable in a magnetomechanical surveillance system, the magnetostrictive element may nevertheless act as an amorphous magnetic element which can still produce harmonic frequencies in response to an electromagnetic interrogating field. This is undesirable because after a purchaser of an item bearing the magnetomechanical marker has had the marker degaussed at the checkout counter, that purchaser may then enter another retail shop where a harmonic EAS system may be in use and where it would be possible for the degaussed marker to set off an alarm because it may generate harmonic frequencies in response to an interrogation signal in the second retail store.
The present inventors have found that when conventional magnetostrictive materials used in a pulsed interrogation system are annealed in the presence of a transverse magnetic field, the ring down characteristic of the materials is adversely affected. The time of ring down is substantially reduced thereby rendering the marker less unique as a magnetomechanical marker.
U.S. Pat. No. 5,252,144, issued to Martis, has proposed that various magnetostrictive materials be annealed to improve the ring down characteristics thereof. However, unlike the present invention, the Martis patent does not disclose applying a magnetic field during heating.